Hydroformylation reactions involve the preparation of oxygenated organic compounds by the reaction of carbon monoxide and hydrogen (a.k.a., syn or synthesis gas) with carbon compounds containing olefinic unsaturation. The reaction is performed in the presence of a carbonylation catalyst and results in the formation of a compound, for example an aidehyde, which has one more carbon atom in its molecular structure than the starting olefinic feedstock. By way of example, higher alcohols may be produced in the so-called "oxo" process by hydroformylation of commercial C.sub.6 -C.sub.12 olefin fractions to an aidehyde-containing oxonation product, which on hydrogenation yields respective C.sub.7 -C.sub.13 saturated alcohols. The crude product of the hydroformylation reaction will contain catalyst, aldehydes, alcohols, unreacted feed, syn gas and by-products.
Before further processing of the crude product is possible, it is necessary to remove the catalyst therefrom. One conventional method of removing cobalt values from such a crude product is set forth in U.S. Pat. No. 4,625,067 (Hanin), which issued on Nov. 25, 1986. The process disclosed in Hanin involves the contacting of the crude product with a stream of stripping gas to entrain volatile cobalt compounds, characterized in that the contacting is performed in the presence of water and aqueous acid to dissolve those cobalt values not entrained in the gas under the conditions of temperature and pressure employed for the contacting, and the aqueous phase is subsequently separated from the organic hydroformylation reaction product.
The Hanin patent has the disadvantage that when lower carbon number olefins (e.g., C.sub.7 and below) are used as the feedstock, unreacted compounds such as olefins and/or paraffins are stripped out together with the volatile cobalt compounds. These olefins and/or paraffins are then absorbed into the olefinic feedstock and recycled to the oxo reactor. This occurs because lower carbon number feedstocks such as heptene have roughly the same volatility as the cobalt specie, thereby causing it to be entrained together with the volatile cobalt and taken out overhead. Light hydrocarbons which are absorbed into the olefinic feedstock rapidly build up within the cobalt recovery system causing an undesirable decrease in net olefin feed rate.
U.S. Pat. No. 5,237,105 (Summerlin), which issued on Aug. 17, 1993, discloses a method of recovering cobalt values which does not cause the build up of unreacted light hydrocarbons within the system, thereby avoiding a decrease in the olefin feed rate. This is accomplished by providing a demetalling step prior to the stripping step which produces a substantially cobalt-free organic hydroformylation reaction product and water soluble cobaltous salt aqueous product. The organic hydroformylation reaction product is diverted for further downstream treatment, while the water soluble cobaltous salt aqueous product is concentrated, converted to cobalt carbonyl and stripped of volatile cobalts which are substantially free of any light hydrocarbons using a recirculating alcohol.
For alcohol grades containing eight or more carbon numbers the catalyst cycles of Hanin and Summerlin are similar. The common feature being a stripper reactor which recovers greater than 70% of the cobalt contained in the oxonation stream overhead where it is absorbed from the gas stream by the entering olefin feed stream. In order to maintain cobalt balance the additional 30% of the cobalt must be produced by taking the aqueous cobalt formate stream from the stripper reactor bottoms converting it to a cobalt carbonyl via a preformer reactor. As the alcohol content decreases and carbon number increases the preforming efficiency of the oxonation product decreases.
The Hanin patent discloses two phase non-catalyzed preforming. In order to decrease the volume of the preformer and to facilitate grade switching the Summerlin patent uses a catalyzed system.
The Summerlin patent also discloses an alternative mode of operation (i.e., the demet mode) which is used for lower carbon number grades. In the demet mode, the oxonation product is decobalted by an air oxidation of the cobalt carbonyls to water soluble cobalt formate. The decobalted oxo product is water washed and sent to the hydrogenation section. The cobalt formate stream is concentrated in an evaporator and mixed with hexyl alcohol before being fed to a preformer reactor. The preformer product is fed to a stripper reactor where the cobalt carbonyls are stripper overhead as in the Cobalt Flash operating mode. The hexyl alcohol bottoms are recycled to the preformer, and the cobalt formate is sent to the demetalling section for trace carbonyl cleanup. This mode of operation requires that 100% of the cobalt carbonyl be generated in the preformer versus 30% in the Cobalt Flash mode. This is feasible since the hexyl alcohol stream is a much more efficient preforming organic than oxonation product.
In both the Hanin and Summerlin patents, the reflux from the stripper reactor is passed to an absorber where it is contacted with a feed olefin for recycling either to the oxo reactor or to the stripper reactor. This reflux consists mainly of water and cobalt carbonyls, containing from 800 to 2600 ppm cobalt. The present inventors have recognized that certain advantages can be gained by diverting at least a portion of the stripper reactor reflux to the preformer reactor instead of the absorber. Stripper reactor reflux which is diverted to the preformer reactor can be used as a preforming reaction initiator. Since the preforming reaction is autocatalytic, the use of stripper reactor reflux as a preformer initiator will speed up the reaction which results in a smaller reactor volume and reduces or eliminates the need for a heterogeneous preforming catalyst.
The present invention offers optimization opportunities for both the Cobalt Flash mode which is disclosed in both the Hanin and Summerlin patents and the demet mode which is disclosed solely in the Summerlin patent.
In accordance with the Cobalt Flash mode as disclosed in the Hanin patent, a two phase non-catalyzed preformer is used. The preformer has an inherent inhibition time when oxo product is used for the preforming organic. The present invention provides for the recycling of the stripper reactor reflux introduces HCo(CO).sub.4 to the preformer reactor, wherein the HCo(CO).sub.4 acts as a reaction initiator so as to greatly reduce or eliminate inhibition time. This will in turn permit the use of a smaller preformer reactor and greatly simplify and speed up olefin grade switching when no other source of cobalt carbonyl is present. This would require a holding drum, etc. for the stripper reactor reflux.
In a catalyzed preformer reactor as disclosed in the Cobalt Flash mode of the Summerlin patent, the incentives are to reduce catalyst costs or to compensate for deactivated catalyst. The recycling of the reflux from the stripper reactor to the preformer has been found to be useful in this mode since it substantially reduces or eliminates the need for a preformer catalyst.
When light grades are hydroformylated under Summerlin's demet mode of operation, hexyl alcohol is used and has been found to be very efficient. For a deactivated catalyst system, however, the recycling of the stripper reactor reflux to the preformer actually improves the operation and perhaps even extends the catalyst's service life.
The present invention also provides many additional advantages which shall become apparent as described below.